The invention is related to methods for detecting CPM-modulated information symbols, as well as apparatus for practicing such methods. More particularly, the invention relates to such a method and apparatus in which the CPM signals are transmitted through an AWGN channel are incoherently demodulated by a frequency discriminator or differential demodulation into a signal which is then sampled with the symbol clock after having passed a whitened matched filter.
When transmitting digital signals, modulation methods are used which allow for reduction of the data rate and thereby allow for improved utilization of the transmission channels. Normally, data are transmitted by carrier frequencies, and at the receiver side, demodulation and decoding, respectively, of the noisy signals is required.
Continuous phase modulation methods (CPM) are mostly used in transmission systems which, apart from good bandwidth efficiency, must also be insensitive to non-linear distortions in the transmission chain. For an unsusceptible system concept it is, in particular with wireless transmission systems, e.g., radio link systems, often necessary that it can tolerate phase and frequency variations of the oscillators (for mixing, frequency shift, etc.) used in the transmission chain without bit errors appearing or even loss of synchronization. Such phase and frequency variations can be caused by temperature variations and/or mechanical effect influences on the oscillators. The system concept should allow such influences without serious degradation of the system properties in terms of bit error rate at a given signal-to-noise ratio.
Until now, coherent detecting methods with carrier restoration have been used, which, while exhibiting good properties concerning noise interference, suffer from loss of synchronization in the transmission system when sufficiently large frequency transitions occur in the superheterodyne oscillators, so as to produce entire bit error blocks. Such synchronization losses are caused when the phase-locked loop as used for carrier restoration becomes unlocked. In order to minimize such frequency transitions appearing in the oscillators, extensive measures will be needed, in particular in the radio link technique (frequencies up to 55 GHz).
Further to the coherent methods, non-coherent detecting methods are known, which either suffer from large degradation of the system properties in terms of noise, as compared to coherent methods (see K.-S. Chung, Generalized Tamed Frequency Modulation and Its application for Mobile Radio Communications, IEEE Journal on Selected Areas in Communications, Vol. SAC-2(4):487-497, July 1984), or which require unrealistically complicated measures for realizing them which cannot be practiced at this time (see A. Svensson, T. Aulin and C.-E. Sundberg, Symbol Error Probability Behaviour for Continuous Phase Modulation with Partially Coherent Detection, International Journal on Electronics and Communications (AExc3x9c), Vol. 40(1):37-45, January 1986). The large degradation of the method according to Chung is caused by the non-optimal conversion from the continuous range into the time-discrete range and to the colored noise at the input of the Viterbi algorithm suggested for correcting the symbol interferences.
The presently most promising attempts are based on the combination of a whitened matched filter (hereinafter called WMF) with succeeding correction of the symbol interferences caused by such filtering (see S. Bellini, M. Sonzogni and G. Tartara, Non-coherent Detection of Tamed Frequency Modulation, IEEE Transactions on Communications, Vol. COM 32(3): 218-224, March 1984, and further S. Bellini and G. Tartara, Efficient Discriminator Detection of Partial-Response Continuous Phase Modulation, IEEE Transactions on Communications, Vol. COM-33(8):883-886, August 1986). A drawback of such a method is that the WMF used can only be approximately calculated for particular frequency pulses. Furthermore, such WMF supplies at its output an infinite pulse which requires extensive measures in the following signal processing.
All of the foregoing articles are fully incorporated herein by reference.
An object of the invention is to provide for a method and an apparatus for detecting CPM-modulated information signals, allowing calculation for any frequency pulse and supplying at its output, in response to a frequency pulse, a minimum-phase finite pulse.
To that end, the invention provides a method and/or apparatus in which the demodulated CPM signal is subject only to partial whitened matched filtering, whereby the signal pulses decrease at the expense of remaining colored noise.
The concept of the invention resides in a modified draft solution of a whitened matched filter which combines the objects of a transition from the time continuum to discrete times without loss of information and the generation of un-correlated (xe2x80x9cwhitexe2x80x9d) noise contributions, at the considered discrete times. The modification is that, for the beginning, the fact is being disregarded that the noise is colored as a consequence of using a frequency detector or a differential phase detector. Such filter, for that reason, later on called to be a partial whitened matched filter, is designed such that
1. the matched filter G*(f) is adapted only to the frequency pulse used, and
2. the whitening filter w(z) cancels the color of the noise caused by the matched filter.
This design is selected such that the partial WMF filter response to a frequency pulse will be a minimum phase pulse. The above mentioned remaining color of the noise interference is canceled as far as possible by means of a further minimum-phase whitening filter F(z) having a freely selectable finite order. This whitening filter F(z) is designed such that for a given filter order, the noise power at its output will be a minimum. Such a system provides, in response to a frequency pulse, an optimally decreasing minimum phase pulse which will be corrected by a typical sequence estimation method.
The additively superimposed noise will be minimal for the freely selectable pulse length. This allows for a simple choice between efficiency and complexity.
The sequence estimation can be performed with a simple sub-optimal method (DFSE, as shown by A. Duel and C. Heegard in their article Delayed Decision-Feedback Sequence Estimation in Proceedings of the 23rd Annual Allerton Conference on Communications, Contr. Comput., October 1985), fully incorporated herein by reference, or it can be performed by a more complex optimal method as, e.g., reported by B. E. Spinnler and J. B. Huber in their article Design of Hyper States for Reduced-State Sequence Estimation in International Journal of Electronics and Communications (AExc3x9c), Vol. 50: Jan. 17-26, 1996, fully incorporated herein by reference. The whitening filter F(z) can be designed in optimal manner for any frequency pulse. The advantages of incoherent detection (reduced requirements as to carrier phase constance) will be retained.
The invention is explained in more detail below in the following detailed description of the presently preferred embodiments with reference to the accompanying drawings.